Printed circuit boards (PCBs), also known as printed wiring boards, are used to interconnect and assemble electronic circuits. The operating temperature, mechanical strength, and other characteristics of a PCB, may vary according to the application in which the PCB is used. PCBs provide electrical conductor paths between the circuits disposed upon them.
Some PCBs consist of paper or woven glass impregnated with an epoxy resin. PCBs may include materials such as copper, iron, aluminum, or ceramic. Flexible PCBs may have polyester or polyimide bases. Ultimately, PCBs typically include at least a resin-based material, a reinforcement material, and one or more conductive foils.
FR-4 is the most common printed circuit board material, and is used in the majority of computer-based applications. The reinforcement material in FR-4 is typically a woven fiberglass material that is impregnated with an epoxy resin, which may vary in composition.
One of the measured characteristics of the PCB is its dielectric constant. The dielectric constant of a material relates to the velocity at which signals travel within the material. The dielectric constant is actually variable, and may change with a modification in frequency, temperature, humidity, and other environmental conditions.
Traces etched or otherwise routed on the PCB carry signals between circuits at a certain speed. The propagation of the signal between the circuits, known as its “time of flight,” is proportional to the length of the trace. Thus, board layout designers typically route straight-line traces between the circuits on the PCB.
The speed of a signal propagating along a trace is inversely proportional to the square root of the dielectric constant of the PCB upon which the trace is formed. Thus, the dielectric constant of the PCB affects the speed of all signals propagating on the PCB. Since most PCBs are not homogeneous, but a blend of materials, the dielectric constant measured on the PCB is slightly different when taken over a strand of woven glass, as compared to a measurement taken between two strands of glass.
The woven glass of the PCB is typically aligned at right angles within the PCB material, forming a familiar weave pattern. For those PCBs that are rectangular or square in shape, the weave pattern within the PCB is thus substantially orthogonal to two sides of the PCB and parallel to two sides, although deviations from this alignment may be found. Likewise, signal traces, such as input/output (I/O) buses, and other electrical interconnect traces, are typically routed in a direction parallel to the sides of the PCB, taking right angle turns where a change in direction is needed.
The routing practices, as well as the material properties of the PCB, result in a condition where traces may be disposed parallel to the glass strands or orthogonal to the glass strands. Those that run in parallel will have a random probability of being routed directly over a parallel glass strand or between a set of parallel glass strands. Even when a trace runs over a glass strand, their relative positions may change, due to a skewing of the underlying glass strands. Those traces that run orthogonal to the glass strands will intermittently be disposed over glass strands all along the trace. These varying conditions make it difficult to ascertain the dielectric constant of the PCB beneath the trace, thus making the signal propagation speed along the trace difficult to successfully predict.
When a trace is disposed over a parallel glass strand, the trace achieves a relatively higher dielectric constant, Dk, than when the trace is disposed between two glass strands. Thus, two traces that are parallel to one another and identical in length on the same printed circuit board may propagate signals at different speeds, based upon the relative position of the underlying material within the PCB. In addition to the speed difference, impedance variations between the traces are likely to occur. These phenomena are particularly troublesome for bus signals, in which multiple traces for each I/O line of the bus, while having identical lengths between circuits, do not have identical impedances and may not propagate at the same speeds. The variation in impedance and speed are dependent on the type and size of the glass weave, the size of the trace, and the orientation of the trace.
Thus, there is a need for a method to route traces such that the dielectric constant can be predicted and, thus, the speed and impedance of signals along the trace can be more accurately known.